There is substantial evidence that microbially generated secondary bile acids are cocarcinogenic, in animal model studies and that these steroids may facilitate the development, growth and spread of colon cancer. The concentration of secondary bile acids in the large bowel appears to be a function of dietary habits and the 7 alpha-dehydroxylating ability of the intestinal microflora. The initial aim of the proposed research is the purification, characterization and regulation of 7 alpha-dehydroxylase from Eubacterium sp. V.P.I. 12708 using affinity chromatography techniques. The second aim is to chemically synthesize 3 alpha, 12 alpha-dihydroxyl-delta 6-5 beta-cholenoic acid and 3 alpha-monohydroxy-delta 6-5 beta-cholenoic acid and to determine if these compounds are intermediates in the 7 alpha-dehydroxylation reaction and if they can be rehydroxylated to form 7 alpha or 7 beta bile acids by whole cells or cell extracts. The third aim will be to determine if the delta 6-cholenoates have mutagenic-carcinogenic activity using the Ames assay and a battery of mammalian cell transformation systems. The fourth aim will be to isolate and characterize bile acid resistant mutant strains of Eubacterium sp. V.P.I. 12708 with respect to 7 alpha-dehydroxylase activity, whole cell protein profiles and phospholipid profiles. The final aim is to isolate an intestinal bacterium capable of converting chenodeoxycholic acid to ursodeoxycholic acid.